1. Field of the Invention
The present invention relates to a metro network and, more particularly, to an optical fiber used in a metro network as a transmission line.
2. Description of the Related Art
In order to meet the demand for higher transmission and reception of data at a high speed, wavelength division multiplexing (WDM) optical communication systems are being implemented. The WDM optical communication systems can be classified into dense wavelength division multiplexing (DWDM) systems and coarse wavelength division multiplexing (CWDM) systems. Further, optical communication networks employed in wavelength division multiplexing optical communication systems can be classified into access networks, metro access networks, metro core networks, long-haul networks, ultra-long haul networks, etc.
The access network interconnects nodes within a distance of 1 km to 5 km; the metro access network interconnects nodes within a distance of 20 km to 100 km; the metro core network interconnects nodes within a distance of 100 km to 300 km; the long-haul network interconnects nodes within a distance of 300 km to 1000 km, and the ultra-long haul network interconnects nodes apart from each other by a distance of at least 1000 km.
The access network and the metro access network typically employ single-mode optical fibers. An optical fiber employed in the metro access network has a negative dispersion characteristic and includes a core having a high refractive index and a clad surrounding the core. The optical fiber may include an annular portion formed between the core and the clad and having a refractive index lower than that of the core.
U.S. Pat. No. 4,715,679 issued to Bhagavatula discloses a single-mode optical waveguide comprising a core having at least one annular region with a depressed refractive index and a clad surrounding the core.
A metro network is more economical in the implementation at a low transmission speed through employing a direct modulation (DM) scheme instead of an external modulation (EM) scheme. The direct modulation scheme typically employs a directly modulated distributed feedback laser (DM-DFB) as a light source for generating an optical signal. The DM-DFB has a positive chirp. Therefore, wavelength dispersion occurs when the light source transmits an optical signal, via an optical fiber having a positive dispersion in the wavelength band of 1550 nm, more than a predetermined distance. The wavelength dispersion may cause a distortion of an optical signal and may cause a noise such as cross talk between optical signals. Therefore, it is necessary for a metro network to have additionally a dispersion compensation fiber module for compensating the dispersion of an optical signal as described above.
Further, the direct modulation-type metro network using an optical fiber having a positive dispersion characteristic must include at least one dispersion compensation fiber module, which complicates the construction of the network and increases cost for installing the network.
In order to avoid the complex construction of the network, there has been proposed a method of constructing a metro network using an optical fiber having a negative dispersion characteristic.
However, this method may have limitations in the transmission distance. FIG. 1 is a graph showing the curves of Q-factors for a typical single-mode fiber (SMF) and typical negative-dispersion fibers (NDFs) according to the transmission distance. Specifically, FIG. 1 shows a Q-factor curve for the transmission of a pre-amplified optical signal having an extinction ratio of 5 dB through a first NDF, a Q-factor curve for the transmission of a pre-amplified optical signal having an extinction ratio of 8 dB through a second NDF, a Q-factor curve for the transmission of an optical signal without being pre-amplified, which has an extinction ratio of 8 dB, through a third NDF, and a Q-factor curve for the transmission of an optical signal without being pre-amplified, which has an extinction ratio of 8 dB, through the SMF. It is difficult for a typical SMF to have a good transmission property due to the chirping during the direct modulation. Also, it is noted that a typical NDF has a limitation in the transmission distance due to its large negative dispersion characteristic.
Recently, in order to minimize the distance limitation, which may be caused in an optical fiber having a positive dispersion characteristic, a method of employing an optical fiber having a dispersion value between −7 and −8 ps/nm/km at a wavelength of 1550 nm has been proposed. The optical fiber having a dispersion value between −7 and −8 ps/nm/km has a dispersion characteristic proper for the C-band area (1530˜1565 nm). Unfortunately, such a large negative dispersion value of the optical fiber, i.e, a dispersion value between −7 and −8 ps/nm/km, makes it difficult to apply the optical fiber to a metro network when the transmission of an optical signal requires a distance range of at least 100 km in an L-band area (1565˜1625 nm) at a speed of 10 Gps.